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Creating Cathodes for Air-Breathing Biobatteries

Wednesday, 01 May 2013

Devices that support various functions of our bodies are
being used increasingly. Today, they include cardiac
pacemakers or hearing aids. Tomorrow, they may be
contact lenses with automatically changing focal length or
computer-controlled displays generating images directly in
the eye. But, none of these devices will work if not coupled to
an efficient and long-lasting power supply source. Researchers
from the Institute of Physical Chemistry of the Polish
Academy of Sciences (IPC PAS) in Warsaw say that the best
solution seems to be miniaturized biofuel cells that consume
substances naturally occurring in the human body or in its
immediate surroundings.

They have developed an efficient electrode to be used in the
construction of biofuel cells or zinc-oxygen biobatteries. After
installation in a cell, the new biocathode generates a voltage,
over many hours, that is higher than what can be obtained in
existing power sources of similar design. The most interesting
feature, they say, is that the device is air-breathing—it works at
full efficiency when it can take in oxygen directly from the air.

Common batteries and rechargeable batteries are unsuitable,
they say, to power implants inside the human body
because they use strong bases or acids. The battery housing
must, therefore, be tightly sealed.

Here is where biofuel cells offer an essential advantage: they
do not require a housing. To get electricity, it is enough to
insert the electrodes into the body, they say.

“One of the most popular experiments in electrochemistry is
to make a battery by sticking appropriately selected electrodes
into a potato. We are doing something similar. The difference
is that we are focusing on biofuel cells and the improvement of
the cathode. And, of course, to have the whole project working,
we’d rather replace the potato with...a human being,” says
Dr. Martin Jönsson-Niedziółka, a research associate in the
Department of Eletrode Processes, IPC PAS.

In the experiments, his group uses zinc-oxygen batteries.
The principle of their operation is not new. Batteries constructed
in this way have been popular since before alkaline power
sources came.

“At present, many laboratories work on glucose-oxygen biofuel
cells. In the best case they generate a voltage of 0.6-0.7 V.
A zinc-oxygen biobattery with our cathode is able to generate
1.75 V for many hours,” says Adrianna Złoczewska, a PhD student
at IPC PAS, whose research has been supported under the
International PhD Projects Programme of the Foundation for
Polish Science. (See Figure 1)

The main component of the biocathode developed at the IPC
PAS is an enzyme surrounded by carbon nanotubes and encapsulated
in a porous structure—a silicate matrix deposited on an
oxygen permeable membrane. Their group has been working
for many years on the techniques needed to construct the cathode
using enzymes, carbon nanotubes, and silicate matrices.

An electrode so constructed is installed in a wall of a small
container. To have the biofuel cell working, it is enough to
pour an electrolyte (here: a solution containing hydrogen
ions) and insert the zinc electrode in the electrolyte. The pores
in the silicate matrix enable oxygen supply from the air and H+
ions from the solution to active centers of the enzyme, where
oxygen reduction takes place. Carbon nanotubes then facilitate
the transport of electrons from the surface of the semipermeable
membrane.

A cell with the new biocathode is able to supply power with
a voltage of 1.6 V, for a minimum one and a half weeks. The
cell efficiency decreases with time, likely because of gradual
deactivation of the enzyme on the biocathode.

In the experiments carried out so far, a stack of four batteries
connected in series successfully powered a lamp composed
of two LEDs. However, before the biofuel cells based on the
design developed at the IPC PAS get commercialized, the
researchers must solve the problem of relatively low electric
power that is common to all types of biofuel cells.

The research being conducted may someday create miniaturized
power supply sources for medical implants, biosensors, or
light-emitting tattoos.

Institute of Physical Chemistry of the Polish Academy of Sciences, Warsaw, Poland
http://www.ichf.edu.pl

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